PD-1 synergizes with other co-inhibitory molecules to effectively terminate T cell responses to control T cell tolerance and tissue inflammation. PD-1 inhibitory signals also contribute to T cell dysfunction in cancer and chronic infections. A deeper understanding of PD-1 signaling is needed to determine how to improve pathogen/tumor immunity, while minimizing autoimmunity and immunopathology. To address these issue, we worked with Core C to generate novel PD-1 signaling domain mutant mice. Mutation of the ITSM motif leads to tumor clearance similar to PD-1 blockade, but surprisingly, ITSM mutant mice develop lower incidence and reduced severity of Experimental Autoimmune Encephalomyelitis (EAE). These data suggest that it may be possible to dissociate beneficial effects of PD-1 pathway blockade on tumor immunity from autoimmunity. Our findings complement those of P1 showing that ITIM and ITSM mutant mice have distinct outcomes in chronic viral infection, and P3 showing distinct outcomes of these strains in cGVHD. Based on these preliminary data, we hypothesize that the PD-1 ITSM and the ITIM signaling motifs may play distinct roles in different T cell subsets and/or within different tissue microenvironments. In addition, we have discovered that exhausted CD8+ T cells in tumors and FoxP3+ Tregs in autoimmune tissue sites highly express a novel inhibitory receptor, CD112R. P1 has found that CD112R is highly expressed on more terminally exhausted T cells with distinctive functional properties during chronic viral infection. P3 has identified distinctive Tregs that highly express CD112R. We hypothesize that CD112R collaborates effectively with PD-1 to regulate effector T cells and Treg expansion and/or function. To test these hypotheses, our specific aims are: Aim 1: To assess the cellular and molecular impact of PD-1 ITIM and ITSM signaling in autoimmunity and anti-tumor immunity. We will use EAE and tumor models to determine impact of these PD-1 mutations on CD8, CD4 Tcon and FoxP3+ Treg cells and RNAseq to determine mechanisms by which these mutations affect effector vs. regulatory T cells responses. Aim 2: Study the cellular and molecular mechanisms by which CD112R regulates autoimmunity and anti-tumor immunity. We will use novel antibodies and CD112R-deficient mice to study the roles of CD112R in effector and Treg cells. We will also investigate if CD112R synergizes or antagonizes PD-1 functions and its differential signaling via ITIM vs. ITSM motifs. Our studies may help identify strategies to uncouple autoimmunity from anti-tumor immunity associated with PD-1 blockade, which will have a major impact in patients treated with check-point blockade therapy.